What is wrong with my ripple BCD down counter circuit?

Question

I want to implement a BCD ripple down-counter in Proteus.

The following is my circuit. The logic is simple. When the "original" 4-bit binary ripple counter reaches 1111 (15), the 2nd and 3rd flip-flops must reset to 0. To this end, I AND the outputs of all flip-flops and feed the output of the AND gate to the reset input of the 2nd and the 3rd flip flop.

But the circuit does not work properly. When I turn the circuit on, the seven-segment display shows the following sequence:

9 --> 8 --> 1 --> 0

And after 0, it goes back to 9 and continues.

By the way, if I cut the output wire of the AND gate, the circuit perfectly counts from 15 to 0.

I have tried another flip-flops such as D and another model of J-K flip-flops; But the problem did not solve.

Please help me. Thanks

Answer 1

This happens because it is a ripple counter and ripple counters will produce glitches that will cause your intermediary reset circuit to reset even when you believe 1111 isn't present. You need to have a better circuit. When the result is settled at 8 and you clock the circuit one more time, you get a series of intermediary values like this: -

• 1000 = 8 (settled result waiting for next clock)
• 1001 at the instant the next clock comes along
• 1011 pretty soon after the above clock occurred
• 1111 a short time later and this resets the circuit producing your error
• 0111 <-- it never gets to 7

As you can see, for each clock, the flip-flops ripple through their results hence, why it's called a ripple counter.

Answer 2

This is happening due to intermediate glitches in your reset circuitry, which is pretty common in combinational logic. When state 1000 is reached, then progressively all the flops will be changing their states . Considering {Q3Q2Q1Q0} as outputs, the transition at output count is given by: 1000 -> 1001 -> 1011 -> 1111 (resets flops).

Now this issue can be resolved using better reset circuitry, such that when output is 0000, then the D pins of required flops can be overriden to make sure that next state is 1001, hence involving some feedback

Thanks